This invention relates generally to security for buildings and in particular to a low cost device useful with both existing and new installations of lock sets on doors of residences and other buildings.
In the vast majority of locked doors forced entry is relatively simple and may be achieved in a variety of ways. The so-called "credit card" key is simply a piece of flexible plastic that engages the cam surface of a spring loaded bolt causing the bolt to retract from the strike plate opening to unlock the door. Metal tools are also used in this same manner. Some lock sets avoid this problem by the use of a dead bolt that has no angled cam surface. This will defeat the usual plastic card approach, but still leaves the door susceptible to the simple procedure of forcibly removing the strike plate by inserting a screwdriver or other tool into the space between the door edge and the jamb to engage the bolt or the strike plate. The strike plate is then torn loose from the jamb by a sharp blow on the tool. With the strike plate loosened it is a simple matter to push the door open even though the lock bolts are still extended. This is so, because the usual door frame jamb is of soft wood, typically pine, and the strike plate is secured thereto only by light screws that are torn loose with relatively light force. Once the strike plate is loosened, the bolt can be pushed through the soft wood which fractures readily. In a few installations, the door and frame are steel, however, a strong screwdriver or similar tool may still be inserted between the door edge and the jamb at the strike plate or the bolt receiving opening in the jamb if no strike plate is used. The tool is then at a proper angle for the application of force to tear or slice the steel whereupon, even though the lock bolts are still extended (lock position), the door may be swung open. Also, if the stiff tool engages the bolt substantially normal thereto, the bolt can sometimes be sheared by a sharp blow.
The type of forced entry just described is favored by burglars because it is fast, requires few tools and can be accomplished with almost no suspicious movement on the part of the burglar.